2 Answers
2

I Googled online for an answer and also asked some people at the local camera store if they had any idea. I came to know that Canon has two 2x FD extenders FD 2X-A and FD 2X-B. Apparently FD 2X-A are for lenses 300mm or higher and FD 2X-B are for lenses below 300mm.

So, I took a risk and bought a FD 2X-A extender from eBay and I was able to attach the extender to my Sigma 400mm lens. I then attached the FD to EOS converter to the extender.

I focused the combination on the moon to see if I could get infinity focus and I was happy when I was able to focus further from the moon.

very low light will reach the sensor (2x extender already "remove" several LUX)

only manula focus and settings (speed)

aperture could be unavailable if there is no settings on the lens itself (normaly there is one on old lens)

no special coating on last lens of the group, so some ghost possible coming from mirorred light by the sensor (not the case with old roll so not needed to be removed from lens)
-change in focla lenght focus (displacement due to FS adapter) so loose of focus distance scope

partial information on EXIF

generally a worst quality of image compare to actual lens (especialy L serie of this focal)